FIG. 2 shows a cross-section of a structure of a Si submont for mounting a semiconductor laser chip according to the prior art. In FIG. 2, reference numeral 1 designates a monocrystalline substrate of Si. Reference numeral 2 designates a barrier layer disposed on each of the upper and lower surfaces of the Si substrate 1 to be in ohmic contact therewith. This barrier layer 2 comprises a layer Ti layer 21, a layer Ni layer 22, and a layer Ag layer 23 successively deposited outwardly from the substrate. Reference numeral 4 designates a solder layer of Pb or Sn (or an alloy layer) produced by evaporation on the barrier layer 2.
The function of this submount for semiconductor laser element will be described.
Generally, when a semiconductor laser element is under continuous oscillation, it generates heat and therefore the laser is fixed to a radiating block (i.e., a heat sink) thereby to radiate heat. Ag or Cu, each having a high thermal conductivity-are widely used as radiation blocks. However, since a semiconductor laser element comprising for example AlGaAs, and Ag or Cu have a large thermal expansion coefficient difference, a submount formed of a Si monocrystal is inserted between the semiconductor laser element and the heat radiation block. This submount is used as heat stress relaxation material.
The process for producing the submount for a semiconductor laser element which is shown in FIG. 2 is the following.
First--Ti layers 21, Ni layers 22, and Ag layers 23 are successively deposited on the front and rear surfaces of the Si wafer substrate 1 by evaporation. This Ti layer 21 adheres well to the Si substrate 1 and can form an ohmic contact with the Si substrate 1. The Ni layer 22 can suppress the metal alloy reaction between the PbSn solder layer 4 and the Ti layer 21. The Ag layer 23 can prevent the oxidation of the Ni layer 22 and provide a good plating property for PbSn solder. These metal layers can also be produced by sputtering. Next, Pb and Sn are respectively evaporated onto the Ag layer 23, or an alloy of Pb and Sn is evaporated onto the Ag layer 23, to produce a solder layer 4.
When a semiconductor laser element is mounted on a heat radiation block with the above-described Si submount, a junction down construction method in which the light emitting point of the semiconductor laser element is arranged close to the submount is utilized to lower the heat resistance. This junction down construction method is becoming a main method used in high power lasers and laser printers in which low heat characteristics are required.
When Au series is used as solder, although it has good adherence, it has a high fusing point and strength. Heat stress distortion is likely to be produced in the semiconductor laser chip after the solder hardens, thereby reducing long period reliability. Accordingly, PbSn series solder which has a low fusing point and is also soft is mainly used as the solder in the junction down structure; since PbSn solder is soft solder, when a semiconductor laser chip is adhered thereby, the life time characteristics of the semiconductor laser (usually called a laser diode) is good. However, there are following problems.
A semiconductor laser has a pair of resonator end surfaces, and laser light is emitted from the front surface and rear surface of the resonator. There is a proportional relationship between the laser light output which is emitted from the front surface and the laser light output which is emitted from the rear surface. Generally, in order to obtain a constant laser light output from the front surface (APC driving), a monitor photodiode which receives light emitted from the rear surface is included in a package to monitor the rear side laser light. In a case of above-described junction down construction, laser light incident on the monitor photodiode, comprises direct light emitted from the rear surface of the laser resonator and light reflected from the surface of the PbSn series solder at the uppermost surface of the submount. In case of PbSn series solder, the reflectance thereof varies with the number of heat cycles as shown in FIG. 3(a). The quantity of light incident on the monitor photodiode changes, thereby disabling APC driving, and an over current is applied to the semiconductor laser element which causes deterioration of the semiconductor laser element.
Furthermore, PbSn series solder is susceptible to thermal fatigue, and to chips have detached due to lack of adhesive strength after at about 100 heat cycles.
In the semiconductor laser element submount of such a construction, the surface state of the solder is likely to change, and the quantity of light incident on the monitor photodiode varies thereby to varying the monitor current, preventing APC driving. Furthermore, PbSn solder is susceptible to thermal fatigue and to chip detachment.